WRN maintains genomic stability and thus protects against development of malignancy, maklng it critical to understand how WRN functions in vivo. Compelling recent results lead to the hypothesis that a key function of WRN is to resolve intermediates produced during recombinational repair. The goal of this project is to test that hypothesis and identify how separate activities of WRN contribute to recombination. To that end, we will pursue three aims. (1) We will identify a conserved motif essential to Holliday junction recognition, and determine the function of this motif in repair of DNA damage in human cells. (2).We will assign specific functions to the separate activities of the WRN polypeptide, and distinguish activities of WRN that are essential and redundant to function. (3) We will determine how WRN collaborates with other factors within the pathway that carries out recombinational repair of DNA breaks. The results of these experiments will provide mechanistic understanding of WRN and of pathways of homologous recombination; identify specific domains and activities of WRN as promising molecular targets for therapeutic intervention; and allow polymorphisms in WRN to be correlated with impairment of function, thereby improving diagnosis of disease susceptibility and treatment of disease.